This invention relates to sizing grates for granular material and more particularly for self-cleaning devices particularly adapted for coal handling. Coal as removed from the mine is for the most part too large for commercial power plant purposes. In order to bring the mine run coal to a commercially acceptable size, it is processed through a coal crusher which reduces the maximum granule size to 0.75 to 1 inch. The granule size of mine run coals varies dramatically and is influenced by such factors as the type of coal, the particular mine, the amount handling and the method of transportation and storage. It is not uncommon for as much as fifty percent of the mine run coal processed through the crusher to be already below the maximum desired size. Further crushing the already small coal granules results in an increase in fines (small or dust-like particles) which cause serious handling problems. Also passing already properly sized coal through the crusher reduces crusher capacity, increases crusher power consumption and more importantly increases the likelihood of the crusher jamming or "mill skid".
While the concept of automatically sizing coal to allow the already properly sized coal to bypass the crusher seems simple, from a practical standpoint, it is an enormously difficult task. A typical coal crusher in a power plant will process one to two million tons of coal per year. Any coal sizing grate system has to be capable of withstanding continuous use in a very harsh, abrasive environment. When grates have been used in the past, it is frequently necessary to shut the machine down and clean the coal lodged in the grate apertures using a spud or wand directing compressed air through the grate openings. This was a time-consuming and labor intensive process making sizing grates impractical and rarely used in coal power plants.
An object of the present invention is to provide a grate for sizing the granular material such as coal or the like and to separate which are less than a predetermined size. Another object of the invention is to provide a sizing grate which will not be clogged and rendered unusable by material granules. Another object of the invention is to provide a durable sizing grate with high reliability and a long useful life.